Process of cracking mineral oil



Dec. 23, 1930.

A. E. PEW, JR.. ET AL PROCESS OF CRACKING MINERAL OL Filed Feb. 24, 1926Patented' Dea 23, 1930v UNITED slrA'rE PATENT4 oFFlcE .an'rrrun n raw,JB., or' BRYN mwa., ann HENRY rnoms, or BIDLEY PABX, rumi- ACGRPORATIONOF NEW' f"` EY L ELPHIA, PENNSYLVANIA,

PROCESS F GBACKUIG MINERAL OIL V.Application lecl February 24, 1926..Serial No. 80,247. i

in the are of treating high hihhg aha high gravity hydrocarbon oils forthe purpose of decomposing them to convert them into lower boiling andlower gravity hydrocarbon oils,-

' 5, which art is generally known as cracking,

m useful industrial application is in the production of gasolnefrom suchhigher boiling and higher gravity petroleumfoils as gas oil and fueloils. The cracking operation may be conducted in batch or it may becarried on in a continuous manner. The cracking may be eHected in vaporphase or in liquid phase or partly in one phase and partly in another.With a suciently high temperature (the minimum of which is about 600 F.)and under a ressure of several atmospheres, 'a considera le percentageof, for example, gas oil, may be decomposed or cracked into gasoline. At800 and higher temperatures, under the moderate pressures specified, theconversion will take place largely in vapor phase, or in both phases. Ifthe pressure be greatly increased, a larger proportion of the oil willbe cracked in liquid phase. At a pressure of (say) 600 pounds to thesquare inch or (say) 40 atmospheres, the oil will be cracked almostwholly in liquid phase.

In continuous processes for cracking higher boiling petroleumconstituents to gasoline,

it is old and well known to convey the oil to be decomposed, which maybe initially preheated, through a long tube' or tubes' in immediatecontact with heating gases and thereinto raise the oil to acracking-,temperature and either complete the cracking therein orperform a part of 'the cracking therein. In the latter case, the partlycracked oil may be conveyed to a container or reaction chamberv ofrelatively large cross-section, 'wherein the pressure is maintained, but

wherein the temperature, although lower, is maintained in the crackingzone. In this chamber the cracking is completed and the oil will berelatively quiescent to allow deposition of the coke which is formedunder the high temperature conditions. Such procgenerally esses havebeen carried on in vapor phase, in paixed phase and in liquid phase'. Inthe liquid phase process, the oil may be allowed to escape through aloaded valve into a va, porizer, wherein there is maintained apressurebut little above atmospheric. After passing the valve, the most of theoil immediately vaporizes. The vapors are fractionated and condensed. Inthe vapor phase process, the cracked, or partially cracked, va.- por istaken off from the container and goes either direct to a condenser, orthrough a fractionating tower in which a ,product of the desired boilingpoint is separated out.

In such processes, the formation of carbon and rcsinous matters givesrise to serious dif- 'Ihe formatlon and' accumulation 'rates thetrouble, in that much of the coke that vwould otherwise'form or depositin the tubes forms or deposits in the reaction chamber orV drum.U Thetubes need to be cleaned less frequently, lbut they must be cleaned atintervals and frequent cleaning of the reaction chamber or drum isnecessary.

The object of the invention is to provide a practicable process ofcracking in vapor and liquid phase in which little or no coke will beformed.

Before describing the process it may be informative to set forth more indetail the cause, nature and effects of coke formation. Certain factorsin the problem are well understood, while other factors ,seem not to beappreciated and may not have been heretofore known.

With the usual method of heating the oil to raise it to a crackingtemperature, whet-her it is flowing in a long tube or comparativelyquiescent in a tank, the temperaraised not only to a decomposition orcrackingtemperature but to a temperature high enough to separate outcertain constituents .and convert them into coke, which is deposited onthe tube wall. Substantial deposition of coke does not occur until afterthe process has been in operation for some little time. Once, however,the coke starts to form, it forms very rapidly. The coke acts as aninsulator, requiring more heat to penetrate it in order to heat theentire body of oil to a cracking temperature. This raises thetemperature of the tubes. The coke that has already formed acts toabsorb oil, which, at the increased temperature of the tubes, causes itto form coke rapidly. In short, once the format-ion of the coke starts,its rate of production is continually accelerated.

Careful experimental work in connection with the practical developmentof the present invention, wherein the oil was heated to a crackingtemperature under conditions which prevented local heating much abovethe upper limit of the desirable cracking temperature, resulted in theformation of little or no coke. That is, coke occurs within temperatureranges that are materially above the upper limit of the desirablecracking temperature zone. In the ordinary cracking processes, the oilflowing through the tubes is necessarily subjected to an excessivelyhigh temperature because the heat transfer from gases of combustion tothe oil is very low and, in a commercial unit, it is essential that thedierence in temperature between the heating gases and the oil be verygreat; otherwise, the large amount of heating surface necessary wouldinvolve too high a cost. Further, the insulating effect of theaccumulating layer of'carbon inthe tubes necessitates a still greatertemperature difference between the furnace gases and the tubes and thesubjection of the peripheral zone of the stream of oil to still highertemperatures in order that the main body of the traveling oil shall besubjected to the lower cracking temperat-ure desired.

However, while, in said experimental and development work, the oil wascracked without substantial formation of coke, asphaltene was formed insubstantial amounts. Asphalteneis often mistaken for coke and, likecoke, it has insulating qualities and, its formation, when started,proceeds at an accelerating rate. Asphaltene, however, unlike coke, issoluble in different liquids, such as carbon bisulphide, or inpetrolene, which latter substance itself may be separated out bydissolving the asphaltene in gasoline, precipitating the asphaltene andevaporating the gasoline from the petrolene. Coke that is formed athigher temperatures cannot be dissolved and must be removed from` thetubes by mechanical means.

It is clear that if the temperature difference between the heatingmedium and the oil could be economically reduced, the conditionspromotive of the formation of coke would be eliminated. It is also clearthat even if the temperature of the heating medium could not be heldbelow a point at which asphaltene will form, and that if the asphalt/eneremains behind and accumulates in the tubes, the latter could be cleanedexpeditiously and economically by simply running through the tubes, atconvenient intervals, a solvent of the asphaltene.

We have found it to be practicable to secure the desired low temperaturedifference between the heating medium and the oil by means of theindirect application ofthe primary heating agent and the directtransmissions of heat to the oil b means of a secondary heating agent wich, under practicable absolute pressures, will boil at a temperature towhich it is desirable to raise the oil,4 and which, in its vapor form,is flowed into a heat exchange relation with the oil and is condensedthereby and gives up its latent heat to the oil, the condensatereturning to a liquidbody of the substance, which is continuously beingheated by the primary heating agent and is continuouslyeneratng vapor.In the application o Pew & Thomas, No. 13,040 filed March 5, 1925, whichis a continuation in part of an application filed March 17, 1924, No.699,615, the advantage of using a vaporized metal, such as mercury, as adirect heating agent for mineral oil, is clearly and fully disclosed.Mercury has a high boiling| point and high heat conductivity; it willnot oxidize or disintegrate when heated or brought into contact withiron; and it may be condensed and its latent heat transferred to thehoil by heat exchan e9e at temperatures within the desirable zone of.oilcracking temperatures. The most important and valuable quality ofmercury vapor, in its application to the cracking of oil, is that, toaccomplish the same degree and rapidity of cracking, the differencebetween the temperature of the mercury vapor and that to which it isnecessary to raise the oil need be very small relatively to thenecessary temperature difference between the oil and ordinary furnac'egases or other ordinary heating media. In other words, the4 temperatureo the heating medium from which heat is directly transferred through thetube wall'to the oil may be so relatively low that all the objectionablecoke forming conditions hereinbefore recited are minimized or avoided.No hard, insoluble, coky or resinous residue is form after the processhas been in operation for some time; but at an time after .it starts toform, the oil crac ing process may be interruptedfor a few minutes, andthe asphaltene may be readily removed by dowin a stream of solventliquid through the tu es. Other positive advantages are hereinafterenumerated, as they may be more readil appreciated after the process isfully descri d.

ln heating oil by means of furnace gases applied direct to a stream ofoil flowing through a tube, a great temperature difterence between thefurnace gases and the oil. is not objectionable while the oil is beingheated up to a cracking temperature. Below about 600 F., heat can beapplied to the oil at a comparatively high rate per square foot ofheating surface without injurious local cracking. lt is therefore foundpracticable to utilize the waste heat of the primary heating medium bysubjecting the relatively cold oil to the heat of the furnace gasesafter they pass beyond the mercury boiler, thereby quickly preheatingthe oil to a high temperature, which, however, preferably does notreach, and ought not to exceed, the temperature at which substantialcracking begins.

ln a patent issued to us November 20, 1928, No. 1,692,786, there is setforth a process and apparatus that are particularly adapted to thecracking of oil inits liquid phase. The object of the present inventionis to set forth a process that will avoid the objections to priorcracking processes and that is particularly adapted to crack the oil intwo phases (liquid and vapor).

While the process is not dependent for its execution on any particularconstruction and arrangement of apparatus, the layout shown in theaccompanying drawing, which is an elevation, in diagram, of a completecracking plant, `is well adapted to carry out the process in aparticularly practicable and advantageous way.

The mercury boiler av is contained in a furnace b which is provided witha down-take c. The furnace gases pass upward around the boiler, thencelaterally and down downtake c and thence upward through a chimney d.

Above thel boiler is arranged a cracking unit comprising a shell efunctioning as an oil heating and mercury vapor-condensing chamber, anda nest of oil carrying tubes ,extending within the shell.

From the mercury boiler a a mercury vapor lineg extends upward tothe-higher end of chamber 3. From the lower end of the charnber is amercury condensatev return line comprising a trap consisting of a cup hand a goose-neck i, another goose-neck la and a pipe-line m leading backto the, boiler a. The upper part of gloose-neck z' connects with theupper part of t e trap by a vapor pipe j.

drum n adapted to be partly filled with oil, has anv inletpipe o fromthe upper end header of the oil tubes f andan outlet pipe p leading to asettling chamber r. From chamber r a pipe s extends to the lower endheader of the tubes f. In the pipe line s is a pump t. Pipe u aiords anoutlet for heavy residue from the settling chamber r.

Vapor from drum n is taken out through pipe lv to a fractionating towerw. The lighter uncondensed products from tower w go to a condenser .Theheavier products may go to a condenser y or, by pipe z, may be returnedto the drum.

Drum n and the cracking unit e. f are heavily insulatedto reduce theheat losses from exposed surfaces.

At the start of the process drum n may be partly illed with oil from anysource of supply. lt desired, the oil may be preheated by circulating itupward, from a supply pipe 8, through a nest of tubes 9 in the down-takec, and thence, through a pipe 10, to drum n, preferably by way of towerwand pipe a.

In a by-pass line 5, connecting the mercury vapor line g with the returnmercurycondensa-te line m, isa safety valve or pressure regulating-valve6 and a condenser 7.

ln the following description of the operation of lthe process, it shouldbe borne in mind that cracking takes place in both. a liquid phase and avapor phase. and that the amount of cracking that will voccur inlAtemperature of 600 F.). Through line 10 the preheated oil .Hows totower w and is thence conveyed to the drum. Oil is circulated from drumn through line p, settling chambery', line a, tubes f and line 0 back tothe drum. i

, The mercury boiler may be operated at a pressure of about 80-95 poundsgauge pressure, which will give 'a mercurv vapor temperature of about900 F. The mercury vapor passes through the chamber e of the crackingunit. By heat exchange with the oil in the tubes f, the vapor iscondensed and its latent heat is transferred to the oil. The mercurythat is condensed in chamber e is returned by gravity to the boiler. Inorder.

lboiler that (assuming that all the mercury skilled engineer.

- perature in the After the oil enters the cyclic oil heater n f, it isgradually raisedv in temperature.

After the operation has continued long enough to establish normalworking conditions, the temperature of the oil in the drum n will befrom 7 50o-800 F., while the temperature of the oil leaving the pipes fwill be substantially hi her, say 875 F. The temrum, however, is acracking temperature, but is necessaril lower than the temperature ofthe oil leaving the tubes: first, because the oil in the tubes receivesdirectly the heat of. the heating medium; second, because of therelatively cold oil that is being fed in through line a; third, becauseofthe vaporizationv taking place in the drum; and fourth', because ofthe cracking reaction which is taking place.

In order to reach the necessary temperature foresubstantial cracking totake place, it is necessary to subject the oil to a ressure of (say) tenatmospheres. The igher the pressure, the greater the pro ortion of theoil that will be cracked in liquid phase. .The higher the temperature,the more rapid the cracking. Undesirably high temperature differencesbetween the mercury and the oil may be avoided by reducing thetemperature of the mercury vapor (which is effected by reducing thepressure in the boiler) and effecting the cracking more slowly. However,no part of the oil, even the peripheral film adjacent the tube wall, canbe raised above the temperature of the mercury vapor, and if the latteris maintained below the temperature at which material amounts of cokewill form, thel tubes cannot become clogged with coke and the oil cannotbe more or less insulated, by such coke, from the heating medium, asoccurs in ordinary cracking processes, in which the temperature of theheating medium is necessarily v ery much higher than the temperature towhich it is desired to heat the oil. p

The heavy unvaporized residual oil will settle in the chamberfr andmaybe drawn off, continuously or intermittently, 'through line u.

Vapor from drum n goes to tower w, Where it is fractionated. The lighterproducts pass in Vapor stage to a condenser either direct or through apurifier (not shown). The heavier condensed products may be returned todrum n -or may be withdrawn from the stem.

The cracking takes place both in tubes f and in the drum n, but, due tothe small amount of oil in the tubes in proportion to the total amountof oil in the cyclic system, the larger proportion of oil is cracked inthe drum. The main function of the tubesis to transfer heat to the oilin the drum and keep such oil at cracking temperature.

The process may be carried on in batch, but it is preferred to carry iton continuously.

Any foreign matter in the mercuryecollects in the trap h z' and is thusprevented from being carried back into the boiler In the crackingsystemdescribed, due to the prevention of localized heating of the oilsubstantially above the highest predeter mined cracking temperature, nopart of the oil reaches the temperature at which coking takes place.There is ultimately some formation of asphaltene, which, at moderatelyhigh temperatures, is formed before the production of coke at stillhigher temperatures, as hereinbefore explained. At any time afterasphaltene starts to form and deposit on the tube, a solvent may bepumped through the oil system, which dissolves the asphaltene and thuscleans out the tubes. This method of cleaning out the tubes, whollyunworkable to clean out coke deposits, and therefore ina plicable toother systems of cracking, is oliviously strikingly simple, eX- pedientand economical. 4

The main advantages of the process above described may be enumerated asfollows:

Little or no heat is wasted in coke formation and no heat is Wasted inpeneratingany heat insulating wall of coke. The latent heat released bythe mercury on condensation is therefore practically all utilized indoing useful work. The cracking operation is therefore conducted with ahigh degree of economy and efficiency.

Inasmuch as the tubes are never subjected to high heat, they never burnand there is no appreciable depreciation of equipment.

To the extent that clogging of the tubes is avoided, the s stem can beoperated continuously for a re atively long time, which is anotherfactor that makes for economy.

'Ihe rate of heat transfer to the oil may be accurately controlled.

No part of the oil is heated above the maxi mum cracking temperaturedesired.

In the preferred mode of practicing the process, the primary heatinrmedium (i. e., furnace gases) is utilized to first heat the secondaryheating medium (the mercury) that directly furnishes the heat requiredfor cracking, and to next preheat the oil to about the lower limit ofthe temperature range for cracking,` thereby providing for the mosteconomical utilization of the heat of the primary heating medium.

lltl

While mercury is preferred as the second- 'a i heating agent', it isossible to substitute oter vaporizable meta ,such as ca and zinc. Y f

We do not herein claim that part oi the described process whichinvolves, in addition to the maintenance, or a tube temperature belowthe zone of coke-forming temper-vv cracking lzone to decompose higherboiling constituents into lower oiling constituents -i'n the secondcracking zone and to continue the decomposition inthe 1| cracg Zone,conveying vapor from the hrst crachng zone to the rractionating zone andtherein separating lighter vapors and withdrawing them and condensingheavier vapors, and re- 'turning theicondensate, together with the rstspecihed intiowing preheated oil, to the owing circuit ci oil undergoingcracking. In testimony oi which invention, we have hereunto set ourhands, at Marcus Hook, Pa, on this 19th day of February, 1926.

v ARTHUR E. PEW, Jn. HENRY THUMAS.

which comprises continuously .llowing the oil from a source of supplyinto a cracking zone and thence continuously circulating it through a.second cracking zone and back to the first cracking zone, vaporizing abody of liquid mercury and nog the mercury vapor into heating relationwith the second cracking zone so as to eiect condensation of mercuryvapor and impart, by such condensation, sumcient heat to the oil in thesecond cracking zone to decoose higher boiling constituents into loweroiling constituentsv in the second'cracking zone and to continue thedecomposition in the first cracking zone, continuously removing vaporsfrom the first crack zone and fractionating the vapors outside thespecied circuit.

2. The process ot cracking mineral oil which comprises continuouslyHowing the oil from a. source of supply into a'cracking zone and thencecontinuously circulatingitthrough a locus of settling, a second crackingzone and back to the first cracmng zone, vaporizing a body of liquidmercury and iiowing the mercury vapor into heating relation with thesecond cracking zone, so as to effect condensation of mercury vapor andimpart, by such condensation, suiiicient heat to the oil in the secondcracking zone toA decompose higher boiling constituents into lowerboiling constituentsin the second cracking zone and to continue thedecomposition in the first cracking zone, continuously removing vapors vfrom the first cracking zone and fractionating the vapors outside thespecified circuit and withdrawing residual oil from. the settling locus.

3. The process of cracking mineral oil which com rises preheating theoil and continuously owing it through a fractionating zone and thenceinto `a cracking zone and thence continuously circulating the oilthrough a second cracking zone and back to the first cracking zone,vaporizing a body of liquid mercury and flowing the mercury vapor intoheating relation with the second cracking zone so as to effectcondensation of mercury vapor and impart, by such condensation,sufhcient heat to the oil in the second

